Optical modulators have been employed for many years in the field of optical communications to accept modulated data in electrical format (typically radio frequency or RF) and transfer the data onto an optical carrier. In a Mach-Zehnder optical modulator 20, as generally shown in FIG. 1 (PRIOR ART), a beamsplitter 22 divides the laser light from an input optical waveguide 24 into two optical beams propagating in parallel waveguides defining optical paths 28A and 28B, at least one of which having a phase modulator in which the refractive index is a function of the strength of the locally applied electric field. In the example shown in FIG. 1, light in both optical paths 28A, 28B undergoes a phase modulation, although in other configurations the refractive index in only one of the optical paths could be modulated with respect to the other. The beams are then recombined by an output optical combiner 26. Changing the electric field on the phase modulating paths determines whether the two beams interfere constructively or destructively when recombined, and thereby controls the amplitude or intensity of the exiting light. In some configurations, the phase of the exiting light can be controlled via a variety of means such as by manipulating the phase modulation signal, or through design.
In the configuration shown in FIG. 1, the modulating electric field is provided by a segmented Travelling Wave Electrode 21 (or TWE) that consists of two or more transmission line conductors 30A, 30B oriented substantially parallel to the optical paths 28A, 28B, and a plurality of pairs of waveguide electrodes 32A, 32B. Each waveguide electrode 32A, 32B is connected to one of the transmission line conductors 30A, 30B via a corresponding tap or bridge conductor 34A, and 34B. Each bridge conductor 34A, 34B branches out of one of the transmission line conductors 30A, 30B in a direction substantially perpendicular to the optical paths 28A, 28B. The transmission line conductors 30A, 30B convey an RF signal along an RF path that is substantially parallel to the optical paths 28A, 28B.
The configuration shown in FIG. 1 can be operated in “push-pull” mode and is typically referred to as a series push-pull travelling wave electrode, after Klein et al., “1.55 μm Mach-Zehnder Modulators on InP for optical 40/80 Gbit/s transmission networks”, OFC/NFOEC 2006, paper TuA2, and described in further detail by R. G. Walker, “High-Speed III-V Semiconductor Intensity Modulators”, IEEE J. Quant. Elect., vol. 27(3), pp. 654-667, 1991. In a series push-pull configuration, a single voltage signal or field is used to phase modulate the interfering signals in the two arms in anti-phase. Each pair of waveguide electrodes 32A, 32B, as shown in FIG. 1, imparts a phase change to the optical wave in the waveguide 28A, 28B and also acts as a pair of capacitors in series and as a load on the main transmission line conductors 30A, 30B.
A travelling wave electrode Mach-Zehnder optical modulator can be driven using either a single RF signal input, or two RF signal inputs in anti-phase. Referring for example to FIG. 1, in a single-ended design the two transmission line conductors 30A, 30B may respectively act as a signal transmission line conductor (S) conveying the input electrical signal, and a ground transmission line conductor (G) connected to a ground reference. This modulator configuration is sometimes referred to as an SG modulator (also known as a coplanar strip). It should be noted that other types of RF drives are known in the optical telecommunications industry, requiring other arrangements of transmission line conductors in the modulator. For example, the prior art includes optical modulators with differential-drive GSGSG and GSSG formats (see for example applicant's U.S. Patent Application published under number U.S. 2013/0209023 (PROSYK) “Mach-Zehnder Optical Modulator Using A Balanced Coplanar Stripline With Lateral Ground Planes”, filed on Feb. 14, 2013).
FIG. 1A (PRIOR ART) is an elevation view of section A of the optical modulator of FIG. 1, showing two pairs of waveguide electrodes 32A, 32B. Each waveguide electrode 32A, 32B extends over a p-i-n junction 36A, 36B, formed within the corresponding waveguide branch. The p-layer 38A, 38B is in contact with the corresponding waveguide electrode 32A, 32B and the n-layer 40A, 40B is in contact with a common conducting backplane 42. The i-layer 39A, 39B contains a series of layers of InGaAsP of varying composition that acts as the waveguiding core. The entire structure extends on a semi-insulating substrate 43.
When an instantaneous change is applied in the voltage difference between the transmission line conductors 30A and 30B, a RF current 44 flows from the highly p-doped contact material 38A beneath waveguide electrode 32A, through the corresponding p-i-n junction 36A and the common conducting backplane 42, and up through the opposite p-i-n junction 36B. The direct current (DC) bias voltage of the backplane 42 is typically fixed by an external DC voltage source (not shown). A simplified electrical diagram of this configuration is shown in FIG. 2. The p-i-n semiconductor layers act as capacitors 46 that are connected in series through the common conducting backplane 42. This series connection halves the required loading capacitance on the main signal transmission line conductor compared to designs with electrically-independent Mach-Zehnder arms, leading to major performance advantages with regards to bandwidth.
The capacitance C of capacitors 46 shown in FIG. 2 is a significant factor in the performance of a series push pull travelling wave electrode Mach Zehnder optical modulator. If a high capacitance can be used, both the drive voltage of the modulator and the length of the chip can be reduced, which provides advantages in terms of power consumption and chip cost, respectively. However, there is a strict limit to the maximum allowed capacitance, as depicted in FIGS. 3, 4A and 4B, and described in the following paragraphs.
FIG. 3 (PRIOR ART) shows a simplified circuit equivalent model of a travelling wave electrode, well known in the art as the “telegrapher” model. The transmission line conductors are described in this model as an infinite cascade of inductive series elements 48, expressed as an inductance per unit length, and capacitive shunt elements 50, expressed as a capacitance per unit length. The average capacitance per unit length of the capacitors 46, which represent the back-to-back p-i-n junctions of the waveguides, can be expressed as a waveguide capacitance 52 that adds in parallel to the shunt capacitance 50 of the transmission line conductors. Since the total capacitance of two capacitors in parallel is the sum of the individual capacitors, the waveguide capacitance 52 directly adds to the total shunt capacitance 50. The shunt capacitance 50, representative of the transmission line electrodes in the absence of connected waveguide electrodes, is sometimes referred to as the “unloaded capacitance” of the travelling wave electrode. The waveguide capacitance 52 added by the waveguide electrodes is sometimes referred to as the “loading capacitance”.
The inductance per unit length L and total capacitance per unit length C uniquely determine the characteristic impedance Z0 and RF modal index nRF of a travelling wave electrode, given by the equations Z0=√{square root over (L/C)} and nRF=v√{square root over (LC)}, where v is the speed of light in vacuum. In other words, a travelling wave electrode can be equivalently described by either (L,C) or by (Z0, nRF). The situation is depicted graphically in FIGS. 4A and 4B as an exemplary two-dimensional mapping.
The values of (Z0, nRF) are generally fixed by the specifications that the Mach Zehnder modulator must meet. For example, as shown in FIG. 4A, it may be necessary to design the travelling wave electrode to have a characteristic impedance Z0 of 50 Ohm, since that is the most common impedance of commercially produced driver amplifiers. Similarly, nRF is fixed by the necessity to maximize the RF bandwidth of the modulator. Bandwidth is maximized when the velocity of the RF mode is matched to the velocity of the optical mode. The velocity of the optical mode is in turn determined by the optical properties of the waveguides; for example, the typical optical group index is approximately 3.75 for modulators constructed from the compound semiconductor indium gallium arsenide phosphide, which fixes nRF to be approximately 3.75, as also shown in FIG. 4A.
In contrast to (Z0, nRF), the parameters (L,C) are determined by the geometrical design of the transmission line conductors and p-i-n junctions of the waveguides. Since Z0 and nRF are both fixed by performance requirements, the values of L and C are also fixed, and therefore there is restricted freedom in choosing the design of the travelling wave electrode. For example, a specified characteristic impedance of Z0=50 Ohms and a RF index of 3.75 require that the travelling wave electrode have an inductance per unit length L of 625 nH/m and a total capacitance per unit length C of 250 pF/m. The (L,C) of the transmission line conductors in the absence of waveguide electrodes and the (L,C) of the travelling wave electrode including the waveguide electrodes are shown in FIG. 4B. The difference in capacitance between the two points is the waveguide or loading capacitance.
Since the transmission line conductors cannot have an unloaded capacitance of less than zero, the requirement in this example that C=250 pF/m places a theoretical limit on the maximum waveguide loading capacitance: it cannot exceed 250 pF/m. In physically realizable transmission devices, the minimum unloaded capacitance is actually a substantial portion of the total capacitance; for example in some implementations a transmission line must have an unloaded capacitance of 125 pF/m to be practical, or about half of the target total of 250 pF/m. The finite unloaded capacitance of the transmission line places a much stricter maximum on the waveguide capacitance, and therefore severely limits the maximum performance of the modulator.
The problem becomes even greater if a higher characteristic impedance Z0 is desired. Similar to a reduction in drive voltage, increasing Z0 is advantageous in that is reduces driver power consumption. However, at a fixed nRF, increased Z0 results in a decreased total capacitance C. For example, another common commercial driver impedance is 100 Ohm, which gives an inductance 1250 nH/m and a capacitance of 125 pF/m when nRF=3.75. Such a low capacitance requirement leaves little room for sufficient loading capacitance.
There is therefore a need for a Mach-Zehnder modulator design which alleviates at least some of the above-mentioned drawbacks.